1. Field of the Invention
The present invention relates to a voltage control type oscillator suited for a mobile radio communication system within a frequency band width of hundreds of MHz to several GHz and, more particularly, to a voltage control type oscillator constructed such that a plurality of parts are mounted on a printed wiring board to form a resonance circuit wherein an oscillation stage and the oscillation frequency of the oscillation stage is varied within a predetermined frequency band range by varying the resonance frequency of a parallel resonance circuit for the resonance circuit on the basis of an inputted control voltage.
2. Background
Referring to FIG. 3, which is a circuit diagram showing the structure of a conventional voltage control type oscillator, the voltage control type oscillator is used with a mobile radio communication system of, for example, a frequency band of 900 MHz. It generally comprises a control terminal C, an output terminal P, a resonance circuit 1 whose resonance frequency varies depending on a control voltage Vc to be impressed upon the control terminal C, an oscillation stage 12 whose oscillation frequency is determined by the resonance frequency of the resonance circuit 1, a buffer stage 3 which amplifies the signal outputted from the oscillator stage 12 and which prevents the variation of the oscillation frequency of the oscillation stage 12 by the load variation and an output matching stage 4 which matches a next stage circuit to be connected to the output terminal P and inhibits higher harmonics. All portions of the voltage control type oscillator shown in FIG. 3 are formed by mounting a plurality of parts (to be described later) on a printed wiring board made of alumina or the like.
The resonance circuit 1 comprises a coupling capacitor C1, a variable capacitance diode VD, a resonance inductor L1 and a resonance capacitor C2. The control voltage Vc impressed upon the control terminal C is applied to the variable capacitance diode VD through a choke coil L2. The control terminal C is grounded in a high frequency mode by means of a high frequency bypass capacitor C3.
The Colpitts oscillation stage 12 comprises an oscillation transistor Q1 as an active element, bias resistors R1-R3, capacitors C4 and C5, a high frequency bypass capacitor C6 and a chip inductor LL to be connected in series between a voltage supply VB and the bias resistor R3. The oscillation stage 12 is connected to the resonance circuit 1 through a coupling capacitor C7. Further, the bias resistor R3 regulates the DC bias of an emitter as a current outputting terminal of the oscillation transistor Q1. The chip inductor LL has an inductance (for example, several tens of nH--several .mu.H) giving rises to an impedance sufficiently higher than that of the bias resistor R3 within a frequency band of 900 MHz.
The Q (the Quality of the reactive circuit, given by the ratio of the reactance to the total resistance of the circuit) of the resonance circuit 1 is most influenced by the impedance of the emitter of the oscillation transistor Q1, which becomes a load on the resonance circuit 1. The higher the impedance of the emitter, the smaller the deterioration of the Q of the resonance circuit 1. At a high frequency, the impedance of the chip inductor LL becomes high so that the impedance of the emitter of the oscillation transistor Q1 becomes high. Accordingly, the load on the resonance circuit 1 becomes large and the deterioration in Q of the resonance circuit 1 is small so that the C/N (Carrier to Noise Ratio) characteristic of the voltage control type oscillator is improved.
The oscillation stage 12 is connected to the buffer stage 3 through a coupling capacitor C8, at a node between two series connected bias resistors R4 and R5, as well as to the gate of buffer transistor Q2. The bias resistors are series connected between the driving power supply terminal B and ground. The emitter of the buffer transistor Q2 is also connected to ground.
The output matching stage 4 comprises a choke coil L3, a coupling capacitor C10, a high frequency bypass capacitor C11 and an output terminal P. A driving power supply terminal B, to which a driving power supply VB for the oscillation transistor Q1 and the buffer transistor Q2 is applied, is grounded in a high frequency mode by means of a high frequency bypass capacitor C12.
By changing the capacitance value of the variable capacitance diode VD in the resonance circuit 1 in proportion to the value of the control voltage Vc, the resonance frequency of the resonance circuit 1 changes, the oscillation stage 12 oscillates at the changed resonance frequency and an output signal is generated from the output terminal P.
Now, in order to improve the C/N characteristic of the voltage control type oscillator, it is ideal that the impedance of the current output terminal of the active element becomes infinite at an oscillation frequency within a predetermined frequency band. For realizing such an ideal state, it is necessary to bring the impedance of the chip inductor LL into an open state within the predetermined frequency band.
However, as shown in FIG. 4, the chip inductor LL can be expressed as an equivalent circuit with respect to a parallel resonance circuit comprising an inductance L, a resistor R and a capacitor C at a high frequency. Further, in the chip inductor LL, the value of the inductance L, and the values of the resistor R and the capacitance C are determined together and if the value of the inductance L is determined, the values of the resistor R and the capacitance C will be automatically determined, making it impossible to change only the value of the capacitance C. Therefore, in the chip inductor LL, there have been many cases in which the resonance frequency of the parallel resonance circuit greatly deviates from the predetermined frequency band so that the impedance at a frequency using the predetermined frequency band becomes relatively small to deviate from its open state. Accordingly, the conventional voltage control type oscillator has had a first problem that even when the chip inductor LL is used, the degree of improvement of the C/N characteristic of the oscillator is small.
Further, the conventional voltage control type oscillator has had a second problem that since it uses a comparatively expensive chip inductor LL, the overall cost of the oscillator becomes high.
In order to solve the above-described first problem, it is considered that the chip inductor LL is connected in parallel to the chip capacitor and the resonance frequency by the connection of the chip inductor LL and the chip capacitor is caused to coincide with a predetermined frequency band. In this case, however, the number of parts increases so that there arises another problem that the voltage control type oscillator becomes large in size.